The present invention relates to a manufacturing process of semiconductor integrated circuit equipment, and more specifically relates to an optical system which is used to form circuit patterns on a substrate, a method for forming patterns on the substrate by the use of the same, and a mask which is used therefor.
The large-scale integration and the realization of high-performance of semiconductor integrated circuits have been promoted by improving the level of microfabrication of circuit patterns. To form circuit patterns, optical lithography is mainly used, wherein light transmitted through a mask containing a circuit pattern thereon is projected onto the substrate to perform exposure through a projection lens. In this process, a diffraction-limited optical system whose wavefront aberration is sufficiently small is used as the above-described projection lens.
On the other hand, due to a demand for higher-level microfabrication in recent years, a technology has been required that enables to form circuit patterns whose feature size is comparable to or less than the wavelength of the light used for exposure. In connection with the demand, it has become a problem that the accuracy of dimensions of transferred patterns and the accuracy of position of the same were deteriorated by wavefront aberrations that remain in the projection lens. These wavefront aberrations are caused by imperfections in designing the projection lens and manufacturing errors in its manufacturing process, and therefore it is difficult to reduce them completely to zero.
However, in order to satisfy the accuracy of pattern which is a prerequisite to attain required device performance, it is necessary to control the amount of wavefront aberrations to a sufficiently small value by realizing high-precision fabrication and adjustment of the projection lens or by counterbalancing the influence of the aberrations of the projection lens with correction of dimensions and/or positions of the patterns on the mask. To perform either method, it is indispensable to first measure the aberrations accurately and comprehend their characteristic. Recently, coma aberrations, among various aberrations, have been considered to exert significant influence on the quality of transferred patterns. Hence different methods have been tried to measure these as outlined below.
As a first method, direct measurement of the wavefront aberrations by interference measurement has been put into practice. However, an interferometer capable of measuring the projection lens for projecting circuit patterns of a semiconductor integrated circuit on the substrate is exceedingly expensive and therefore its usage is extremely limited.
On the other hand, a method was reported that evaluated coma aberrations by measuring the shift in positions of the image of a modified, deformed guide mark pattern (i.e. so-called box-in-box type), but this method can measure only the influence of third-order coma aberrations in longitudinal and transverse directions. The method for measuring the aberrations by using the shift in positions of the image of the guide mark was discussed, for example, in "Optical Microlithography XI", Proceeding of SPIE, Vol. 3334(1998), pp. 297-308.
Furthermore, a method using an attenuating phase-shifting mask containing an octagonal opening was proposed. This method enables the estimation of third-order coma aberrations by examining a relationship among the amount of exposure at which sidelobes emerge at the vicinity of a projected image of an attenuating phase-shifting mask containing the octagonal opening, their asymmetry, and third-order coma aberrations. The half-tone phase-shift mask was discussed, for example, in Innovation of ULSI lithography technology (in Japanese)", Science Forum Co. Ltd. (1994), p.38. Furthermore, a method for measuring third-order coma aberrations using this mask was presented, for example, in Extended Abstracts for the 58th Autumn Meeting of the Japan Society of Applied Physics, Vol. 2(1997), p.681.
The wavefront aberrations which have hitherto come into question (hereinafter referred to only as aberrations for simplicity) were mainly low-order and/or third-order aberrations. However, with the demand for finer microfabrication of circuit patterns, the accuracy of dimensions and the accuracy of position of patterns which are required became more stringent, and accordingly higher-order aberrations arose as problems to be solved. That is, experimental results in such finer microfabrication could not be well explained only by third-order aberrations, and there grows a necessity to take into account the so-called fifth-order aberration terms. For example, regarding the above-described coma aberrations, the components having a dependency of 3.theta. (.theta.:directional angle of radius vector) in the radial angle direction among the fifth-order aberration terms have become particular problems.
However, it has been, so far, difficult to measure and comprehend these higher-order aberrations easily and quantitatively by means of any of the above-described methods. Accordingly it is hard to obtain a definite guideline about how to adjust the projection lens. In addition to this, the influence of the aberrations on a circuit pattern, to implement a countermeasure therefor, has not been capable of evaluation. As a result, higher-order aberrations became main factors that deteriorate functions and accuracy of semiconductor integrated circuit equipment and decrease the yield of integrated circuits in a manufacturing process with this equipment.